Seasonal variation of inter-hemispheric field-aligned currents deduced from time-series analysis of the equatorial geomagnetic field data during solar cycle 23\u201324

Abstract

The east–west component of magnetic field variation (∆D-component) at Davao station (Philippines, geomagnetic latitude: – 2.22˚N) are used to investigate the characteristics of the long-term Inter-Hemispheric Field-Aligned Currents (IHFACs) based on the time-series analysis from August 1998 to July 2020. Recent in situ satellite and ground-based observations have reported that dusk-side current polarity of IHFAC is often opposite to that of the noon IHFAC, being inconsistent with Fukushima's IHFACs model. We investigated the consistency of the dusk-side IHFAC polarity derived from the observations with the polarity expected from Fukushima’s IHFACs model and examined the solar cycle dependence of IHFACs. It was confirmed that the dusk-side IHFACs during June and December solstices flow in the same direction of the noontime IHFACs, which was consistent with the IHFAC polarities suggested by the Fukushima model. The dusk-side IHFACs around March and September–November months disagreed with the Fukushima model. The ∆D variations clearly showed seasonal asymmetry in the dawn and noon sectors, whereas the ∆D variations in the dusk sector demonstrated seasonal symmetry. Solar cycle dependence of IHFACs was exhibited in the dusk sector. For the dawn and noon sectors, the yearly peak-to-peak ∆D amplitude in the later solar cycle SC24 decreased by about 35% in comparison with the earlier solar cycle SC23. In contrast, the dusk-side yearly peak-to-peak ∆D amplitude increased by about 200%. The dusk-side IHFAC yearly amplitude tended to be in inverse proportion to solar activity.